SUMO fusion protein expression system for producing native proteins

ABSTRACT

A simple and efficient SUMO fusion protein expression system for producing native proteins.

RELATED APPLICATION

This application claims priority to U.S. Provisional Application No. 61/050,663, filed on May 6, 2008, the content of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Fusion protein technology, i.e., expressing in a host cell a target protein fused with a protein partner, allows for enhanced expression of the target protein, which is protected from degradation/mis-folding, easy to be purified/detected, and has improved solubility. The protein partner usually would interfere with the structural or functional properties of the target protein and therefore needs to be removed via, e.g., chemical or enzymatical cleavage, from a fusion protein to generate a free target protein. Cleavage of the protein partner remains the major disadvantage in conventional fusion technology as imprecise cleavage, which occurs frequently, results in failure to recover an active or structurally intact target protein.

The newly developed small-ubiquitin-related modifier (SUMO) fusion technology makes up for this disadvantage. In a SUMO fusion expression system, the fusion partner, i.e., a SUMO protein, can be precisely removed by a SUMO protease, thereby generating a native target protein. See, e.g., Butt et al., Protein Expression and Purification 43:1-9 (2005) and Mossessova et al., Molecular Cell 5:865-876 (2000).

SUMMARY OF THE INVENTION

The present invention relates to an improved SUMO fusion protein expression system that allows directional cloning of any target gene efficiently and producing a target protein having the exact amino acid sequence encoded by the target gene.

One aspect of this invention is an expression vector containing a first portion including a first nucleotide sequence that encodes a Smt3 protein, the 3′ end of the first nucleotide sequence being replaced with a Sfo I site, and a second portion including another restriction site (e.g., XhoI, EcoRI, or BamHI). After inserting a DNA fragment, which or a portion of which encodes a protein of interest, into this expression vector via the Sfo I and the other restriction site, it expresses in a host cell a fusion protein containing, from the N-terminus to the C-terminus, the Smt3 protein and the protein of interest. Cleavage of the fusion protein by U1p1 protease generates the protein of interest. This expression vector can further contain a nucleotide sequence encoding a protein tag (e.g., hexa-His, Maltose binding protein, N-utilizing substance A, Thioredoxin (Trx), Calmodulin-binding protein, Glutathione S-transferase, and α-factor) such that, after inserting the DNA fragment mentioned above, it expresses a fusion protein containing, from the N-terminus to the C-terminus, the protein tag, the Smt3 protein, and the protein of interest. The expression vector described above can be either linear or circular.

The term “restriction site” used herein refers to a nucleotide sequence recognizable by a restriction enzyme, or a nucleotide sequence generated by digestion of a restriction enzyme. “U1p1 protease” is a polypeptide having the protease activity of Saccharomyces cerevisiae U1p1 protease. It can be a full-length Saccharomyces cerevisiae U1p1 protease or a fragment thereof (e.g., residues 403-621) that possesses protease activity, or a fusion protein containing the full-length or a fragment thereof, and a protein tag (e.g., a His-tag).

Another aspect of this invention is a method of producing a protein of interest by: (i) inserting into any of the expression vectors described above the DNA fragment also described above via the Sfo I restriction site and the other restriction site to form an expression construct, (ii) introducing the expression construct into a host cell, and (iii) expressing in the host cell a fusion protein containing, from the C-terminus to the N-terminus, the protein of interest, the Smt3 protein, and optionally, a protein tag. Preferably, the DNA fragment has a 5′ end Gly codon (e.g., GGC) directly linked to a nucleotide sequence encoding the protein of interest. It can be prepared by sticky-end PCR so that no restriction enzyme digestion is needed before inserting it into the expression vector. The fusion protein thus produced can then be isolated via, e.g., affinity column, and subjected to U1p1 protease cleavage to generate the protein of interest encoded exactly by its coding sequence, which preferably has the start codon ATG at its 5′ end.

The term “producing a protein of interest” used herein refers to producing a protein of interest in either free form or fusion form. An “expression vector” is a plasmid containing, among other elements, a highly active promoter and one or more cloning sites downstream of the promoter. This plasmid is used to introduce into and express in a host cell a target gene inserted into the plasmid via the cloning sites. Cloning a target gene into an expression vector produces an expression construct.

Also within the scope of this invention is a method of purifying a protein of interest by: (i) providing a sample containing a fusion protein of, from the N-terminus to the C-terminus, a protein tag (e.g., hexa-His), a Smt3 protein, and a protein of interest, (ii) loading the sample to a protein-tag affinity column (e.g., Ni-NTA column) to allow binding of the fusion protein to the column, (iii) incubating the column with U1p1 protease, which cleaves the fusion protein to produce the protein of interest, and (iv) eluting the protein of interest from the column.

The details of one or more embodiments of the invention are set forth in the description below. Other features or advantages of the present invention will be apparent from the following drawings and detailed description of several embodiments, and also from the appending claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are first described.

FIG. 1 is a photograph showing fusion proteins Trx-RecA and His₆-Smt3-RecA and the RecA protein in free form generated by cleaving the fusion proteins with factor Xa or His₆-U1p1₄₀₃₋₆₂₁-His₆ in SDS-PAGE gels stained with Coomassie-blue. The molecular weight standards are shown on the left.

FIG. 2 is a diagram showing the process of generating a new SUMO fusion protein expression vector pHD.

FIG. 3 is a photograph showing fusion proteins His₆-Smt3-Rad51, His₆-Smt3-HFDV-VP 1, and His₆-Smt3-FMD-VP3 expressed from expression vectors pHD-Rad51, pHD-HFDV-VP1, and pHD-FMD-VP3 in SDS-PAGE gels stained with Coomassie-blue. N: whole cell lysates derived from uninduced cells; I: whole cell lysate derived from IPTG-induced cells; and S: soluble proteins derived from IPTG-induced cells.

DETAILED DESCRIPTION OF THE INVENTION

Described herein is a simple and efficient SUMO fusion protein expression system for producing a target protein having the exact amino acid sequence encoded by a coding sequence.

Described herein is a simple and efficient SUMO fusion protein expression system for producing a target protein having the exact amino acid sequence encoded by a coding sequence. This system utilizes an expression vector containing a nucleotide sequence encoding a Smt3 protein. A Smt3 protein can be the Saccharomyces cerevisiae Smt3 protein, the amino acid sequence of which is shown below:

(SEQ ID NO: 1) Amino acid sequence of Saccharomyces cerevisiae Smt3: MSDSEVNQEAKPEVKPEVKPETHINLKVSDGSSEIFFKIKKTTPLRRLME AFAKRQGKEMDSLRFLYDGIRIQADQTPEDLDMEDNDIIEAHREQIGG A Smt3 protein can also be a functional variant of the yeast Smt3 mentioned above, which is a polypeptide that shares a high sequence homology with Smt3 (e.g., sequence identity at least 85%, 90%, 95%, 98%, or 99%). When fused with a target protein, a functional variant of yeast Smt3 can be cleaved by U1p1 protease to generate a free Smt3 protein having the mature C-terminus of yeast Smt3, i.e., -Gly-Gly at the C-terminus of the free Smt3 protein. See Mossessova et al., Mol. Cell 5:865-876 (2000). In another example, a Smt3 protein is a fusion protein containing yeast Smt3 or its functional variant and a small protein tag, e.g., a hexa-His tag. The amino acid sequence of a His-tag fused yeast Smt3 protein is shown below:

(SEQ ID NO: 2) Amino acid sequence of His-tag-Saccharomyces cerevisiae Smt3 fusion protein: MGSSHHHHHHSSGLVPRGSASMSDSEVNQEAKPEVKPEVKPETHINLKVS DGSSEIFFKIKKTTPLRRLMEAFAKRQGKEMDSLRFLYDGIRIQADQTPE DLDMEDNDIIEAHREQIGG

To prepare the expression vector of this invention, the 3′ end of the nucleotide sequence that encodes a Smt3 protein is replaced with a Sfo I restriction site for cloning downstream thereof a DNA fragment that encodes a target protein. In one example, the 3′ end of the Smt3 coding sequence, i.e., GGTGGT (encoding Gly-Gly), is replaced with GGCGCC (a Sfo I site, encoding Gly-Ala). In another example, the GGTGGT sequence is replaced with GGTGGCGCC (encoding Gly-Gly-Ala). Preferably, this expression vector also includes a nucleotide sequence encoding a protein tag (e.g., His-tag) linked to the 5′ end of the nucleotide sequence encoding Smt3.

A DNA fragment described above, coding for a target protein, can be prepared by conventional methods. Preferably, the DNA fragment is produced by sticky-end PCR such that it can be inserted into the expression vector mentioned above without being digested by restriction enzymes. After inserting the DNA fragment into the expression vector, the junction region of the Smt3 coding sequence and the DNA fragment can have the sequence of GGCGGCATG (encoding -Gly-Gly-M-), in which ATG is the start codon of the protein encoded by the DNA fragment. The resultant expression construct expresses in a host cell a fusion protein containing the Smt3 protein and the target protein, and preferably, a protein tag. The fusion protein can be purified by, e.g., affinity column and then cleaved by a U1p1 protease, which precisely cleaves after the -Gly-Gly- residues, thereby yielding the target protein having the exact amino acid sequence corresponding to its coding sequence. Alternatively, U1p1 protease cleavage can be performed when the fusion protein is still bound to the affinity column.

Several embodiments of this invention are described in the following examples and also in Lee et al, Protein Science, 17(7):1241-1248 (2008).

Without further elaboration, it is believed that one skilled in the art can, based on the above description, utilize the present invention to its fullest extent. The following specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. All publications cited herein are incorporated by reference.

EXAMPLE 1 Production of Fusion Protein Containing Smt3 and Escherichia coli RecA Protein

The Saccharomyces cerevisiae Smt3 gene was cloned into pET32-Xa/LIC vector (Novagen, USA), downstream of the His₆-tag contained in this vector to produce a His₆-Smt3 expression vector.

The open reading frame of the Escherichia coli RecA protein was first cloned into the pET32-Xa/LIC vector (Novagen, USA) to generate a thioredoxin(Trx)-RecA expression construct. The DNA fragment encoding the Trx protein was then replaced by that of the His₆-Smt3 protein. As a result, the open reading frame of the Escherichia coli RecA protein is downstream of and in-frame with the His₆-Smt3 gene to produce a SUMO-RecA expression construct (pSUMO-RecA).

pSUMO-RecA as described above was transformed into JM109(DE3)-competent cells, which were cultured overnight at 37° C. in the presence of 100 mg/L ampicillin to produce an overnight culture (15 mL). The overnight culture was then transferred to 1 L fresh Luria-Bertani medium, grew at 37° C. until it reached an OD₆₀₀ value of about 0.5-0.6, and IPTG (1 mM) was then added to the E. coli culture to induce protein expression. The induced cells were grown at 20° C. for 12 h, harvested, and then centrifuged at 9,000×g for 30 min. The cell pellet thus obtained were lyzed according to the method described in Wang et al., J. Biol. Chem 268:26049-26051 (1993), except that a different lysis buffer [50 mM Tris-HCl (pH 7.4), 300 mM NaCl, 0.2 mM EGTA (pH 8.0)] was used here to prevent non-specific association of His₆-Smt3-RecA with bacterial DNA. After centrifugation, the soluble fraction thus obtained was mixed with 2 mL of Ni²⁺ resins (Amersham, USA) to which the His₆-Smt3-RecA fusion protein binds. The Ni²⁺ resins were washed three times with 30 mL of wash buffer [50 mM Tris-HCl (pH 7.4), 300 mM NaCl, 0.2 mM EGTA (pH 8.0), 40 mM imidazole (pH 8.0)] and the fusion protein bound to them were then eluted.

The His6-Smt3-RecA fusion protein and the free RecA protein released from the fusion protein via U1p1 protease cleavage were detected on an SDS-PAGE gel stained with Coomassie-blue. See FIG. 1. A single band representing free RecA was detected, indicating that U1p1 protease cleaved at only one site in the His6-Smt3-RecA fusion protein. Fusion protein Trx-RecA, expressed from pET32-Xa/LIC-Trx-RecA expression vector, and free RecA released from this fusion protein via factor Xa cleavage, were also detected by SDS-PAGE analysis. Multiple bands representing the released RecA proteins were detected, indicating that factor Xa cleavage took place at unexpected sites in the Trx-RecA fusion protein. See also FIG. 1.

EXAMPLE 2 Preparation of Free RecA protein from His₆-Smt3-RecA Fusion Protein Via a One-Column Approach

Described below is a one-column approach to produce free RecA protein from the His₆-Smt3-RecA fusion proteins produced by the method described in Example 1 by U1p1 cleavage.

U1p 1₄₀₃₋₆₂₁, a fragment of Saccharomyces cerevisiae U1p1 protein (amino acid residues 403-621), has been shown to cleave a C-terminal tagged yeast Smt3 in vitro, producing its mature form (i.e., C-terminal “Gly-Gly). See Mossessova et al., Mol. Cell 5:865-876 (2000). The open reading frame of U1p1₄₀₃₋₆₂₁ was cloned into the pET28a vector (Novagen, USA) to generate an expression vector, which was then transformed into E. coli cells to express a His₆-U1p1₄₀₃₋₆₂₁-His₆ fusion protein. This recombinant enzyme, soluble in water, was purified from the crude extract of the transformed E. coli cells, using Ni²⁺ resins. The final yield was ˜20 mg/L Escherichia coli culture. The protein migrated as a single band on an SDS-PAGE gel stained with Coomassie blue, and with >99% purity as determined by densitometry. The His₆-U1p1₄₀₃₋₆₂₁-His₆ fusion protein exhibited a high affinity to Ni²⁺-resin and did not release from Ni²⁺-resins unless more than 300 mM imidazole or 100 mM EDTA was added to an elution buffer.

An E. coli crude extract containing the His₆-Smt3-RecA fusion protein described in Example 1 was loaded to a column containing Ni²⁺-resins, which were then washed three times with 30 mL of the wash buffer also described in Example 1. Without elution, the Ni²⁺-column, bound with the His₆-Smt3-RecA fusion protein, was then loaded with His₆-U1p1₄₀₃₋₆₂₁-His₆ to allow proteolytic cleavage of the His₆-Smt3-RecA fusion protein. The free RecA protein thus generated was then eluted from the Ni²⁺-resins. The final yield was ˜10 mg proteins per liter of cell culture.

The N-terminus of the eluted free RecA protein was examined by Edman degradation to confirm that the U1p1 cleavage was precise. Results thus obtained indicate that the N-terminal sequence of the RecA protein is identical to that encoded by its gene. The molecular weight of the purified RecA, determined by mass spectrometry, was around 37,843 Da, which is very close to the predicted molecular weight of native RecA protein, i.e. 37,842 Da. This purified RecA protein was shown under electron microscopy to form helical filaments on a circular ds-ΦX174 substrate, indicating that the purified RecA proteins have no apparent polymerization defect.

The biological activities of the purified RecA protein was tested as follows. First, an electrophoretic mobility shift assay was performed to examine whether the RecA protein was capable of binding to DNA, following the method described in Chen et al., Proc. Natl. Acad Sci USA 101:10572-10577 (2004). Incubation of 10 μM RecA with 8 μM (in bp) of ds-ΦX174 DNA resulted in a substantial decrease in the electrophoretic mobility of ds-ΦX174. Six oligonucleotides, i.e., (CT)₂₀, (CA)₂₀, (GT)₂₀, (GA)₂₀, (AT)₂₀, and (CG)₂₀ described in Biet et al., Nucleic Acids Res 27:596-600 (1999), were used to determine the preference of the RecA protein for binding to double-strand (ds) or single-strand (ss) DNAs. 80 μM of each of the oligonucleotides was incubated with the RecA protein in the presence of circular ds-ΦX174 (8 μM) DNA and then a mobility shift assay was performed to examine the binding between the RecA protein and the ds- or ss-DNAs. Results thus obtained show that all six oligonucleotides competed against ds-ΦX174 for binding to the RecA protein. The purified RecA protein exhibited no obvious preference for binding to ssDNA than to dsDNA, as addition of oligonucleotide (CT)₂₀ at low concentrations (2, 4, or 8 μM) resulted no significant increase in the electrophoretic mobility of ds-ΦX174.

Second, a D-loop formation assay was performed to determine whether the purified RecA protein could promote a homology-dependent strand exchange reaction, a bioactivity possesses by native RecA. Briefly, the purified RecA protein (1 μM) was preincubated with 3 μM (in nucleotides) 5′ ³²P end-labeled P1656 ssDNA for 5 min at 37° C. in the presence of 1 mM magnesium acetate and 2 mM AMP-PNP. A D-loop formation reaction was initiated by the addition of an equal volume (10 μL) of a solution containing a supercoiled double-stranded (ds) DNA plasmid GW1 (20 μM in base pairs). See Chen et al. Nucleic Acids Res. 35:1787-1801 (2007); Chen et al., PLos ONE 2:e858 (2007); and Chen et al., Proc Natl Acad Sci USA 101:10572-10577 (2004). Five minutes later, the reaction was terminated by addition of 2 μL SDS (5.5%) and proteinase K (6 mg/ml) for 5 min to remove the proteins. The DNAs contained in the reaction mixture were detected by electrophoresis for 2 h at 4 V/cm on a 0.8% agarose gel in Tris-acetate-EDTA buffer (40 mM Tris, 1 mM Na₂-EDTA, and 20 mM acetic acid, pH 8.0). A phosphorimage of the agarose gel was taken to show the D-loop formation in the presence of RecA proteins. See Lee et al., Biochem Biophys Res. Commun 323:845-851 (2004). Results thus obtained indicate that the purified RecA, just like native RecA, promoted D-loop formation.

Finally, the ssDNA-activated ATPase activity of the purified RecA protein was tested following the method described in Lee et al., Biochem Biophys Res Commun 323:845-851 (2004). In the presence or absence of ss-ΦX174 DNA, the RecA protein released ³²P inorganic phosphate from γ-³²P-ATP, indicating that it possessed the ssDNA-activated ATPase activity.

In sum, the RecA protein produced in the SUMO fusion system described herein and purified by the one-column approach also described herein possessed identical physical and biological features to the native RecA protein.

EXAMPLE 3 Construction of SUMO Fusion Protein Expression Vector pHD

pHD, a SUMO fusion protein expression vector, was constructed as illustrated in FIG. 2. First, the pSUMO-RecA vector described in Example 1 was subjected to five rounds of site-directed mutagenesis reactions to mutate the four Sfo I (5′GGCGCC3′) restriction sites in the backbone of pET32-Xa/LIC to either 5′GGCTCC3′ or 5′GGCACC3′, and to create a new Sfo I restriction site at the SUMO protease cleavage site by mutating “GGTGGT,” encoding the two C-terminal residues ‘GlyGly’ of Smt3 to “GGCGCC”, encoding ‘GlyAla’. Next, the mutated pSUMO-RecA vector thus produced was subjected to Sfo I and XhoI digestion to remove the DNA fragment encoding RecA, resulting in a linear vector pHD, one end of which is a Sfo I site and the other end of which is a XhoI site.

This linear vector can be inserted with any target gene prepared by the sticky-end PCR cloning method described in Shih et al. Protein Sci. 11:1714-1719 (2002) via the Sfo I site and the XhoI site. As illustrated in the right panel of FIG. 2, a target gene can be amplified by sticky-end PCR cloning technology, which requires two PCR reactions in two separate tubes. Both PCR products are purified and mixed equally. After denaturation and renaturation, 50% of the final products carry one Sfo I blunt end and one XhoI cohesive end, and are ready for ligation even without restriction digestion of the PCR products. After ligating the linear vector pHD with the DNA fragment prepared by sticky-end PCR, a new “GlyGly” SUMO cleavage site is generated right before the first amino acid codon of the target gene (see FIG. 2).

EXAMPLE 4 Producing Rad51 VP 1 and VP3 Proteins Using Expression Vector pHD

Rad 51, capsid protein VP1 of Hand-foot-and-mouth disease virus EV71 (HFMDV-VP1), and capsid protein VP3 of foot-and-mouth disease virus (FMDV-VP3) are all known to be very poorly expressed in E. coli using conventional recombinant technology. See Van Komen et al., Methods in Enzymol. 408:445-462 (2006) and also see FIG. 3, panel C.

Genes encoding the three proteins mentioned above were first amplified by sticky-end PCR and then inserted into vector pHD via the Sfo I and XhoI restriction sites. The expression vectors thus formed were transformed into host E. coli cells, in which soluble fusion proteins His₆-Smt3-Rad51, His₆-Smt3-VP1, and His₆-Smt3-VP3 were expressed. See FIG. 3, panel A. These fusion proteins were subjected to cleavage by His₆-U1p1₄₀₃₋₆₂₁-His₆. See also FIG. 3, panel B.

The final yield of Rad51 was ˜10 mg per liter of Escherichia coli culture. Determined by mass spectrometry, this recombinant protein has a molecule weight of 42,964 Da, very close to the predicted molecule weight of native Rad51, i.e., 42,963 Da. Edman degradation analysis confirmed that the Rad51 protein produced in the SUMO fusion system described herein has the exact N-terminus amino acid residue as its native counterpart.

HFMDV-VP1 and FMDV-VP3, when expressed as fusion proteins with Hexa-His tag only, were insoluble. See FIG. 3, panel C. When expressed in the SUMO fusion system described herein, both His₆-Smt3-VP1 and His₆-Smt3-VP3 proteins were soluble. See FIG. 3, panel A. Authentic VP1 and Vp3 proteins were produced after cleavage of the fusion proteins by His₆-U1p1₄₀₃₋₆₂₁-His₆. See FIG. 3, Panel B. Edman degradation analysis confirmed that the N-termini of purified HFDV-VP1 and FMDV-VP3 were identical to those of the native HFDV-VP1 and FMDV-VP3. Mass spectrometry analysis revealed that the molecular weights of HFMDV-VP1 and FMDV-VP3 were 32,829 Da and 23,816 Da, respectively. The predicted molecular weights of these two proteins are 32,744 Da and 23,817 Da.

OTHER EMBODIMENTS

All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the claims. 

1. An expression vector comprising: a first portion including a first nucleotide sequence that encodes a small ubiquitin-related modifier protein (Smt3) having an amino acid sequence at least 85% identical to SEQ ID NO:1, the 3′ end of the first nucleotide sequence being replaced with a Sfo I restriction site; and a second portion including another restriction site, wherein the expression vector, upon insertion of a DNA fragment encoding a protein of interest via the Sfo I restriction site and said another restriction site, expresses a fusion protein containing, from the N-terminus to the C-terminus, the Smt3 protein and the protein of interest, and cleaving the fusion protein by Ub1-specific protease 1 (U1p1) produces the protein of interest.
 2. The expression vector of claim 1, wherein the first portion further includes a second nucleotide sequence that encodes a protein tag, and wherein, the expression vector, upon insertion of a DNA fragment encoding a protein of interest via the Sfo I restriction site and said another restriction site, expresses a fusion protein containing, from the N-terminus to the C-terminus, the protein tag, the Smt3 protein, and the protein of interest.
 3. The expression vector of claim 2, wherein the protein tag is selected from the group consisting of hexa-His, Maltose binding protein, N-utilizing substance A, Thioredoxin, Calmodulin-binding protein, Glutathione S-transferase, and α-factor.
 4. The expression vector of claim 3, wherein the protein tag is hexa-His.
 5. The expression vector of claim 2, wherein said another restriction site is XhoI.
 6. The expression vector of claim 2, wherein the vector is a linear molecule having one end being the Sfo I restriction site and the other end being said another restriction site.
 7. The expression vector of claim 1, wherein said another restriction site is XhoI.
 8. The expression vector of claim 1, wherein the vector is a linear molecule having one end being the Sfo I restriction site and the other end being said another restriction site.
 9. The expression vector of claim 1, wherein the Smt3 has an amino acid sequence at least 90% identical to SEQ ID NO:1.
 10. The expression vector of claim 9, wherein the Smt3 has an amino acid sequence at least 95% identical to SEQ ID NO:1.
 11. The expression vector of claim 10, wherein the Smt3 has the amino acid sequence of SEQ ID NO:1.
 12. The expression vector of claim 1, wherein the expression vector further comprises the DNA fragment inserted via the Sfo I restriction site and said another restriction site.
 13. A method of producing a protein of interest, comprising: providing the expression vector of claim 1, in which a DNA fragment is inserted via the Sfo I restriction site and said another restriction site, the DNA fragment encoding a protein of interest, introducing the expression construct into a host cell, and producing in the host cell a fusion protein containing, from the N-terminus to the C-terminus, the Smt3 protein, and the protein of interest.
 14. The method of claim 13, wherein the DNA fragment contains a 5′ end Gly codon directly linked to a nucleotide sequence that encodes the protein of interest.
 15. The method of claim 14, wherein the DNA fragment has a 5′ end sequence GGCATG, in which GGC is the Gly codon and ATG is the 5′ end of the nucleotide sequence encoding the protein of interest.
 16. The method of claim 14, wherein the DNA fragment is prepared by sticky-end PCR.
 17. The method of claim 13, wherein said another restriction site is XhoI.
 18. The method of claim 13, further comprising isolating the fusion protein from the host cell and cleaving it with Ub1-specific protease 1 (U1p1), thereby producing the protein of interest.
 19. The method of claim 18, wherein the U1p1 protease is His₆-U1p1₄₀₃₋₆₂₁-His₆.
 20. The method of claim 13, wherein the expression vector further includes a second nucleotide sequence encoding a protein tag such that the fusion protein expressed from the expression vector contains the protein tag at its N-terminus.
 21. The method of claim 20, wherein the protein tag is selected from the group consisting of hexa-His, Maltose binding protein, N-utilizing substance A, Thioredoxin, Calmodulin-binding protein, Glutathione S-transferase, and α-factor.
 22. The method of claim 20, further comprising: loading a sample prepared from the host cell that contains the fusion protein to a protein-tag affinity column to allow binding of the fusion protein to the column, incubating the column with Ub1-specific protease 1 (U1p1), which cleaves the fusion protein to produce the protein of interest, and eluting the protein of interest from the column.
 23. The method of claim 22, wherein the protein tag is hexa-His and the protein-tag affinity column is a Ni-NTA column. 